interpret: project_downcast: do not ICE for uninhabited variants

compiler/rustc_const_eval/src/interpret/operand.rs

@@ -260,8 +260,12 @@ impl<'tcx, Prov: Provenance> ImmTy<'tcx, Prov> {
         // This makes several assumptions about what layouts we will encounter; we match what
         // codegen does as good as we can (see `extract_field` in `rustc_codegen_ssa/src/mir/operand.rs`).
         let inner_val: Immediate<_> = match (**self, self.layout.abi) {
-            // if the entire value is uninit, then so is the field (can happen in ConstProp)
+            // If the entire value is uninit, then so is the field (can happen in ConstProp).
             (Immediate::Uninit, _) => Immediate::Uninit,
+            // If the field is uninhabited, we can forget the data (can happen in ConstProp).
+            // `enum S { A(!), B, C }` is an example of an enum with Scalar layout that
+            // has an `Uninhabited` variant, which means this case is possible.
+            _ if layout.abi.is_uninhabited() => Immediate::Uninit,
             // the field contains no information, can be left uninit
             // (Scalar/ScalarPair can contain even aligned ZST, not just 1-ZST)
             _ if layout.is_zst() => Immediate::Uninit,

compiler/rustc_const_eval/src/interpret/projection.rs

@@ -201,25 +201,8 @@ where
         // see https://github.com/rust-lang/rust/issues/93688#issuecomment-1032929496.)
         // So we just "offset" by 0.
         let layout = base.layout().for_variant(self, variant);
-        // In the future we might want to allow this to permit code like this:
-        // (this is a Rust/MIR pseudocode mix)
-        // ```
-        // enum Option2 {
-        //   Some(i32, !),
-        //   None,
-        // }
-        //
-        // fn panic() -> ! { panic!() }
-        //
-        // let x: Option2;
-        // x.Some.0 = 42;
-        // x.Some.1 = panic();
-        // SetDiscriminant(x, Some);
-        // ```
-        // However, for now we don't generate such MIR, and this check here *has* found real
-        // bugs (see https://github.com/rust-lang/rust/issues/115145), so we will keep rejecting
-        // it.
-        assert!(!layout.abi.is_uninhabited());
+        // This variant may in fact be uninhabited.
+        // See <https://github.com/rust-lang/rust/issues/120337>.
 
         // This cannot be `transmute` as variants *can* have a smaller size than the entire enum.
         base.offset(Size::ZERO, layout, self)

compiler/rustc_middle/src/mir/syntax.rs

@@ -1074,6 +1074,8 @@ pub enum ProjectionElem<V, T> {
     /// "Downcast" to a variant of an enum or a coroutine.
     ///
     /// The included Symbol is the name of the variant, used for printing MIR.
+    ///
+    /// This operation itself is never UB, all it does is change the type of the place.
     Downcast(Option<Symbol>, VariantIdx),
 
     /// Like an explicit cast from an opaque type to a concrete type, but without

compiler/rustc_mir_transform/src/dataflow_const_prop.rs

@@ -403,12 +403,7 @@ impl<'a, 'tcx> ConstAnalysis<'a, 'tcx> {
             operand,
             &mut |elem, op| match elem {
                 TrackElem::Field(idx) => self.ecx.project_field(op, idx.as_usize()).ok(),
-                TrackElem::Variant(idx) => {
-                    if op.layout.for_variant(&self.ecx, idx).abi.is_uninhabited() {
-                        return None;
-                    }
-                    self.ecx.project_downcast(op, idx).ok()
-                }
+                TrackElem::Variant(idx) => self.ecx.project_downcast(op, idx).ok(),
                 TrackElem::Discriminant => {
                     let variant = self.ecx.read_discriminant(op).ok()?;
                     let discr_value =

src/tools/miri/tests/pass/issues/issue-120337-irrefutable-let-ice.rs

@@ -0,0 +1,16 @@
+// Validation stops the test before the ICE we used to hit
+//@compile-flags: -Zmiri-disable-validation
+
+#![feature(never_type)]
+#[derive(Copy, Clone)]
+pub enum E {
+    A(!),
+}
+pub union U {
+    u: (),
+    e: E,
+}
+
+fn main() {
+    let E::A(ref _a) = unsafe { &(&U { u: () }).e };
+}

tests/mir-opt/gvn_uninhabited.f.GVN.panic-abort.diff

@@ -0,0 +1,34 @@
+- // MIR for `f` before GVN
++ // MIR for `f` after GVN
+
+  fn f() -> u32 {
+      let mut _0: u32;
+      let _1: u32;
+      let mut _2: E;
+      let mut _3: &U;
+      let _4: U;
+      scope 1 {
+          debug i => _1;
+      }
+      scope 2 {
+          let mut _5: &U;
+      }
+
+      bb0: {
+          StorageLive(_2);
+          StorageLive(_3);
+          _5 = const _;
+          _3 = &(*_5);
+          _2 = ((*_3).1: E);
+          StorageLive(_1);
+-         _1 = ((_2 as A).1: u32);
++         _1 = const 0_u32;
+          StorageDead(_3);
+          StorageDead(_2);
+-         _0 = _1;
++         _0 = const 0_u32;
+          StorageDead(_1);
+          return;
+      }
+  }
+

tests/mir-opt/gvn_uninhabited.f.GVN.panic-unwind.diff

@@ -0,0 +1,34 @@
+- // MIR for `f` before GVN
++ // MIR for `f` after GVN
+
+  fn f() -> u32 {
+      let mut _0: u32;
+      let _1: u32;
+      let mut _2: E;
+      let mut _3: &U;
+      let _4: U;
+      scope 1 {
+          debug i => _1;
+      }
+      scope 2 {
+          let mut _5: &U;
+      }
+
+      bb0: {
+          StorageLive(_2);
+          StorageLive(_3);
+          _5 = const _;
+          _3 = &(*_5);
+          _2 = ((*_3).1: E);
+          StorageLive(_1);
+-         _1 = ((_2 as A).1: u32);
++         _1 = const 0_u32;
+          StorageDead(_3);
+          StorageDead(_2);
+-         _0 = _1;
++         _0 = const 0_u32;
+          StorageDead(_1);
+          return;
+      }
+  }
+

tests/mir-opt/gvn_uninhabited.rs

@@ -0,0 +1,24 @@
+// unit-test: GVN
+// compile-flags: -O
+// EMIT_MIR_FOR_EACH_PANIC_STRATEGY
+// skip-filecheck
+
+#![feature(never_type)]
+
+#[derive(Copy, Clone)]
+pub enum E {
+    A(!, u32),
+}
+
+pub union U {
+    i: u32,
+    e: E,
+}
+
+// EMIT_MIR gvn_uninhabited.f.GVN.diff
+pub const fn f() -> u32 {
+    let E::A(_, i) = unsafe { (&U { i: 0 }).e };
+    i
+}
+
+fn main() {}

tests/ui/consts/let-irrefutable-pattern-ice-120337.rs

@@ -0,0 +1,10 @@
+// check-pass
+#![feature(never_type)]
+#[derive(Copy, Clone)]
+pub enum E { A(!), }
+pub union U { u: (), e: E, }
+pub const C: () = {
+    let E::A(ref a) = unsafe { &(&U { u: () }).e};
+};
+
+fn main() {}